Age and formation of the presumed Late Pliocene to Middle Pleistocene Mühlbach formation, High Rhine Valley, southwest Germany

Abstract. The drainage system of the Swiss Alps shifted from an eastward (Danubian) to a westward (Rhenian) direction during the Late Pliocene. Since then, the High Rhine Valley has acted as the major drainage path during both ice-free and glaciated periods in the past. The Mühlbach formation comprises the patchy remains of carbonate-free sediments that have been interpreted as an equivalent of Sundgau gravel. The Sundau gravel reflects remains of fluvial deposits related to the Late Pliocene drainage of the northern Swiss Alps through the Burgundian Gate towards the Bresse–Rhône Graben. However, the limited data available characterising the Mühlbach formation have led to opposing interpretations regarding its origin and age. Presented here are new data on the distribution and physical appearance of the formation, together with luminescence dating of deposits from the type locality. Sedimentological and petrographic analyses imply that the various deposits assigned to the Mühlbach formation do not represent a genetically coherent formation. In addition, consistent results of quartz and feldspar luminescence dating place deposition of sediments at the type locality at ca. 55 ka. This dates their deposition to the end of a pronounced cold period during the Late Pleistocene that has been shown to be characterised by slope processes at other sites. Altogether, the data presented imply that the Mühlbach formation is not uniform but rather comprised of mainly reworked weathered residues from different time periods.